1. Technical Field
The present invention relates to apparatus and methods for analysis of blood samples in general, and for the determination of the cell volume and mean cell volume of red blood cells within a sample in particular.
2. Background Information
Physicians, veterinarians and scientists have examined human and animals' biologic fluids, especially blood, in order to determine constituent particulate quantities as well as to identify the presence of unusual particulates not seen in healthy subjects. The particulates generally measured, quantified and identified include red blood cells (RBCs), white blood cells (WBCs), and platelets. RBC analyses can include determinations of RBC number, size, volume, shape, hemoglobin content and concentration, and the hematocrit (also referred to as the packed cell volume).
Known blood examination techniques, described in detail medical texts such as Wintrobe's Clinical Hematology 12th Edition, generally divide the examination methods into manual, centrifugal, and impedance type methods. Manual methods for cell enumeration typically involve the creation of an accurately determined volume of a blood or fluid sample that is quantitatively diluted and visually counted in a counting chamber. Manual examination methods include examining a peripheral smear where the relative amounts of the particulate types are determined by visual inspection. Centrifugal examination methods involve centrifuging the sample, causing the sample to separate into constituent layers according to the relative densities of the constituents. Impedance methods involve the examination of an accurate volume of blood which is treated according to the particulate being measured; e.g., lysing RBCs for enumeration of the nucleated cells and volumetrically diluting the sample in a conductive fluid. The process typically involves monitoring a current or voltage applied to sample passing through a narrow passage to determine the effect particles have on the current/voltage as the particles pass through in single file.
All of the aforementioned methods, other than the peripheral smear or centrifugal separation, require dispensing a precise volume of sample. Inaccuracies in the sample volume will result in quantitative errors of the same magnitude in the associated analysis. With the exception of centrifugal methods, all of the aforementioned methods also require the sample to be mixed with one or more liquid reagents or diluents, and also require calibration of the instrument to obtain accurate results. In the case of peripheral smears, a high degree of training is needed to properly examine the smear. A number of the aforementioned methods generate large volumes of contaminated waste which is expensive to handle.